Gas systems for compressing process gas are widely used in industry. In such circumstances, for example in the case of separating CO2 from the exhaust gas of combustion processes, a plurality of compressors or compressor stages are arranged in series, wherein the intake side of each successive compressor stage is connected to the pressure side of the preceding compressor stage. As a result, greater compression can be achieved than with only one compressor stage. Cooling units, such as for example heat exchangers, can be arranged in the gas system between the individual compressor stages, in order to cool the flow of gas—which has a raised temperature after compression—again before the next compression stage. However, highly compressed process gas also makes high demands on the gas tightness of the gas system. Points where a shaft, to which the compressor rotors are attached, passes through the pressure-bearing housing enclosing the rotors are possible sources of leakage. The process gas issuing from this leakage source is captured and fed back into the flow of gas in the gas system. In so doing, care must be taken to ensure that the pressure of the captured process gas from the leakage is greater than the pressure of the process gas at that location of the gas system where the captured process gas is fed back into the gas system. In order to further minimize leaks in this region, the outer spaces around the shaft passages are enclosed by a further housing inside which is a seal gas. N2 or air can for example be used as seal gas in this context. This seal gas is under such a high pressure that it is impossible for the process gas to flow out through the shaft passages. However, it can be advantageous not to entirely prevent the leakage of process gas at the shaft passages. It is known to capture the mixture of seal gas and process gas. The captured mixture is then released into the environment or, if for example the process gas and/or the seal gas is toxic and/or flammable, is disposed of in a separate process. The process gas contained in the gas mixture is thereby lost.